Cold hardiness & minimum temperature

Is Welsh Onion (Allium fistulosum)cold hardy? Hardiness zone & min temp

Also called Bunching onion, Scallion, Spring onion, Japanese bunching onion.

More about welsh onion

About Welsh Onion

Allium fistulosum · also called Bunching onion, Scallion · edible

Welsh onion (Allium fistulosum) is a hardy perennial bunching allium grown for its hollow blue-green leaves and mild scallion-like stems. Unlike bulb onions it forms clumps rather than swelling bulbs, regrowing year after year. Sow in full sun, harvest leaves continuously, and divide established clumps every few seasons. It overwinters reliably and is extremely cold-tolerant.

Cold limit: USDA 4-9 (perennial) · RHS H7 (7-24°C)

Watch for — Crown rot from wet soil: Yellowing, collapsing clumps in waterlogged ground; plant in free-draining soil or raised beds and avoid winter sogginess.

What welsh onion's hardiness rating actually means

Yes — welsh onion is genuinely cold hardy. Rated RHS H7 and USDA 4-9 (perennial), it lives outdoors all year and needs winter cold rather than protection from it. Its RHS rating of H7 means: Hardy in the severest European continental winters. On the US scale that maps to USDA 4-9 (perennial) — the zones where it can be left outdoors year-round.

New to these scales? The USDA hardiness zone map explained covers how the zone numbers work, and you can find your own zone with the zone finder.

Minimum temperature — and what happens below it

Minimum survivable temperature is roughly below about −20 °C. Welsh Onion is built for winter — once established it takes hard frost and snow in its stride.

Concretely, for welsh onion as it gets too cold:

It tolerates winter lows to about −20 °C once established.
Below its rated zone, the visible damage is browned or blackened top growth and, in the worst case, a killed crown or root.
First-year, newly planted, or container-grown specimens are noticeably less hardy than established garden plants — the roots are exposed.

Can welsh onion go outside or overwinter — and where?

Plant it out within USDA 4-9 (perennial) and it overwinters with little or no help.
It does not want to come indoors — a warm winter room actually weakens a hardy plant by denying it dormancy.
The real risks in its range are waterlogging, wind-rock on young plants, and a late hard frost on new growth — not ordinary winter cold.

Work back from your local frost dates with the frost-date calculator: the last spring frost and first autumn frost are what really decide when welsh onion can be outside. US growers can check USDA zones; UK growers should use the RHS hardiness ratings, which match the H7 figure above.

Welsh Onion hardiness — frequently asked questions

Is welsh onion cold hardy?

Yes — welsh onion is genuinely cold hardy. Rated RHS H7 and USDA 4-9 (perennial), it lives outdoors all year and needs winter cold rather than protection from it. An outdoor plant. Welsh Onion is hardy across USDA 4-9 (perennial); it belongs in the ground or a frost-proof container, not on a windowsill, and many types actively need a cold winter to perform.

What is the minimum temperature welsh onion can survive?

Minimum survivable temperature is roughly below about −20 °C. Welsh Onion is built for winter — once established it takes hard frost and snow in its stride.

What hardiness zone is welsh onion?

Welsh Onion is rated USDA 4-9 (perennial) and RHS H7 — Hardy in the severest European continental winters.

Can welsh onion survive winter outside?

Plant it out within USDA 4-9 (perennial) and it overwinters with little or no help. It does not want to come indoors — a warm winter room actually weakens a hardy plant by denying it dormancy. The real risks in its range are waterlogging, wind-rock on young plants, and a late hard frost on new growth — not ordinary winter cold.

What happens to welsh onion below its minimum temperature?

It tolerates winter lows to about −20 °C once established. Below its rated zone, the visible damage is browned or blackened top growth and, in the worst case, a killed crown or root. First-year, newly planted, or container-grown specimens are noticeably less hardy than established garden plants — the roots are exposed.